Wear-resistant, partially uncoated steel parts and methods of producing same

ABSTRACT

A wear-resistant steel part may be formed by hot forming and/or hardening a hardenable steel grade semifinished part. The steel part may be used, for example, as a processing, conveying, and/or crushing mechanism in agricultural machines, conveying machines, mining machines, or building machines. The semifinished part may be heated to a temperature above an Ac1 transformation temperature and then subsequently hot formed and/or hardened. The steel part may be particularly suitable for use with abrasive materials. To that end, the steel part may have at least one region that has been hardened to a depth of not more than 100 microns by surface hardening before the semifinished part is hot formed or hardened.”

The invention relates to a wear-resistant, at least partly uncoatedsteel part consisting of a hardenable steel grade which has beenproduced from a semifinished part by hot forming and/or hardening. Inaddition, the invention relates to a process for producing awear-resistant, at least partly uncoated processing, conveying and/orcrushing means of agricultural machines, conveying machines, miningmachines or building machines from a semifinished part, in which thesemifinished part is heated to a temperature above the Ac1transformation temperature and is subsequently hot formed and/orhardened.

Wear-resistant, at least partly uncoated steel parts which have to havehigh strengths and at the same time are subjected to abrasive forces arerequired, for example, for the production of agricultural machines, inparticular plows, and also for buckets of a dredge or conveying screwsfor abrasive materials, for example the conveying screw of a concretemixer. In order to achieve the necessary high strengths in theabovementioned applications, the parts are preferably subjected to hotforming in which the semifinished parts from which the steel parts areproduced are firstly heated to a temperature above the Ac1transformation temperature point, so that transformation hardening ofthe microstructure is effected by hot forming and subsequent hardening,i.e. rapid cooling, and a material having a martensitic microstructureis formed. The martensitic microstructure has a significantly greaterhardness but also a significantly greater mechanical strength, forexample tensile strength. Corresponding steel parts are known, forexample, from the German patent DE 10 2010 050 499 B3. The German patentdescribes a process for producing dredger buckets, concrete mixerconveying screws, conveying screw blades or other transport blades ofconveying plants, in which the components are hot formed and presshardened.

However, it has been found that the components produced in this way haveproblems in respect of the wear resistance despite the hardening processduring production, especially on contact with abrasive materials.

The German first publication DE 10 2010 017 354 A1 is concerned with theproblem of hot forming of zinc-plated flat steel products to producehigh-strength or very high-strength steel components. When the meltingpoint of the metal of the protective coating is exceeded, there is arisk of “liquid metal embrittlement” which is caused by penetration ofthe molten metal of the coating into the notches or cracks arising informing of the flat steel product. The liquid metal which has penetratedinto the steel substrate deposits at grain boundaries and there reducesthe maximum tensile or compressive stress which can be withstood. As asolution, the patent publication offers nitriding of the outer layerregions, so as to produce finely structured outer layer regions.

The present invention is, in contrast, concerned with the problem thathot-formed and/or hardened steel parts do not have the desired wearresistance in the uncoated regions and are therefore not optimallysuited for use as conveying means, for example on contact with abrasivematerials. It is therefore an object of the present invention to proposeat least partly uncoated steel parts having improved suitability for usewith abrasive materials. In addition an inexpensive production processfor corresponding steel parts should be proposed.

The object indicated is achieved, for a steel part, the steel part atleast partially having a surface region which has been hardened to adepth of not more than 100 μm, preferably to a depth of up to 40 μm, bysurface hardening before hot forming and/or hardening.

It has been found that the heating of the semifinished parts forproduction of the steel parts to a temperature above the Ac1transformation temperature or above the Ac3 temperature before hotforming and/or hardening leads to decarburization of regions close tothe surface, so that the carbon content of these regions issignificantly lower than the carbon content of the base material. As aresult, the region close to the surface up to a depth of 100 μm, inparticular the region up to a depth of 40 μm, cannot be hardened to therequired degree during hot forming and/or hardening. However, it hasbeen found that at least partial surface hardening of the uncoatedregions of the semifinished parts before hot forming and/or hardening togive the steel part leads to both the surface region and the basematerial having very high hardness despite the decarburization of theregions close to the surface as a result of the high temperatures duringhot forming or hardening. This provides a steel part of which at leastpartially has a surface region which has been hardened to a depth ofpreferably 100 μm or in the region down to a depth of 40 μm and istherefore significantly more wear resistant than the at least partlyuncoated steel parts known hitherto.

In a first embodiment, the hardened surface region of the steel part ishardened by carburization or nitriding. Both processes offer theopportunity of hardening regions close to the surface of the steel partin a targeted manner before hot forming or hardening. In addition,nitriding has the advantage that the hardness is not reduced during hotforming. In the case of carburization, the carbon content in the surfaceregions is increased but decreases again due to hot forming.

In a further embodiment, after hot forming and/or hardening the hardenedsurface region of the steel part preferably has at least the hardness ofthe base material of the steel part located under the surface region.

The wear resistance of the steel part can preferably also be improved bythe hardness of the surface region of the steel part being greater thanthe hardness of the base material. It has been found that, inparticular, the hardness of the surface regions is responsible for thewear resistance of the steel part on contact with highly abrasivematerials, so that a very wear-resistant steel part can be produced evenwhen using a somewhat softer base material.

Consequently, the steel part is, according to a further embodiment ofthe steel part, configured for use as processing, conveying and/orcrushing means in agricultural machines, conveying machines, miningmachines or building machines, with at least the regions of the steelpart which are subjected to abrasive forces being surface-hardened.

In addition, manganese-boron steels, dual-phase steels or TRIP steels,in which particularly pronounced martensite formation or transformationof residual austenitic components into martensite makes an increase inthe hardnesses possible, are also particularly advantageous.

In a further embodiment of the steel part, the surface region of thesteel part which has been hardened before hot forming and/or hardeninghas, at least in regions, a hardness of from 400 to 700 HV. These valuesare generally achieved only by very high-strength steel grades after hotforming or hardening in the base material. The surface hardening beforehot forming or hardening offers, in particular, the opportunity ofproviding the starting material for production of the steel componentson a coil.

According to further teaching of the present invention, theabovementioned object is achieved by a process for producing awear-resistant, at least partly uncoated steel part for processing,conveying and/or crushing means of agricultural machines, conveyingmachines, mining machines or building machines from a semifinished part,in which the semifinished part is heated, at least in regions, to atemperature above the Ac1 transformation temperature and is subsequentlyhot formed and/or hardened, in that the semifinished part at leastpartially is subjected to surface hardening in which a surface region ishardened to a depth of not more than 100 μm before hot forming and/orhardening. Preference is given to hardening a surface region having adepth of up to 40 μm, in which decarburization processes usually takeplace during hot forming. The depth of the surface region which is to behardened is controlled by the duration of the hardening treatment. Ithas been found, in particular, that despite heating to a temperatureabove the Ac1 transformation temperature point, the surface-hardenedregions of the steel part remain stable in respect of the surfacehardness, so that high surface hardness can be achieved after hotforming and/or hardening. This leads to the steel parts of processing,conveying and/or crushing means of agricultural machines, conveyingmachines, mining machines or building machines which are in contact withabrasive materials displaying reduced wear.

The hardening of the surface regions before hot forming or beforehardening makes it possible to carry out the surface hardening oncoilable materials, i.e. on steel strip, so that particularly economicalproduction of wear-resistant, at least partly uncoated steel parts fromsemifinished parts is made possible. In a preferred embodiment of theprocess, hardening of the surface region is effected by nitriding or bycarburization. Both processes make it possible to provide a higherhardness in the surface region, which after hot forming and/or afterhardening make a higher wear resistance of the surface of the hot-formedor hardened steel part possible.

The surface hardening is, in a further embodiment, particularlypreferably carried out by a heat treatment in a heat treatmentatmosphere comprising up to 25% by volume of H₂, 0.1-10% by volume ofNH₃, H₂O and a balance N₂ and also unavoidable impurities at a holdingtemperature of from 600° C. to 900° C. The dew point of the heattreatment atmosphere is preferably in the range from −50° C. to −5° C.,so that the effect of atmospheric moisture on the hardening process isreduced. In addition, preference is given to a maximum of 10% by volumeof H₂ and a maximum of 5% by volume of NH₃ being permitted and the dewpoint being set to a dew point temperature of from −40° C. to −15° C. ata temperature of from 680 to 840° C. The latter process parameters gaveimproved and more uniform surface hardening.

The depth of the surface hardening can be set via the time for which theholding temperature is maintained. The time for which the semifinishedpart has the holding temperature during surface hardening is preferablyset to from 5 s to 600 s, preferably from 30 s to 120 s.

The surface hardening is preferably carried out in a continuoushardening furnace, so that, for example, a strip-like semifinished part,i.e. a coilable semifinished part, is also surface-hardened and can befed to the further hot forming and/or press hardening steps. However,surface hardening in a chamber furnace is also conceivable.

As indicated above, semifinished parts such as manganese-boron steels,dual-phase steels and TRIP steels firstly display a particularly highstrength increase during hot forming or during hardening and secondlyprovide the opportunity of bringing the regions close to the surface toidentical hardness in the range from 400 to 700 HV by nitriding. As aresult, steel parts which are very wear-resistant and have particularlyhigh strengths can be produced inexpensively.

In the following, the invention will be illustrated with the aid ofexamples in conjunction with the drawing. In the drawing,

FIG. 1 schematically shows an example of the process for producing awear-resistant, at least partly uncoated steel part,

FIG. 2 shows the layer structure of the semifinished part or steel parttreated as per the example in FIG. 1 in a schematic illustration,

FIGS. 3, 4 shows examples of a steel part for agricultural machines andconveying machines and

FIG. 5 shows a graph of the hardness profile as a function of thedistance from the surface for two examples and a comparative example.

FIG. 1 firstly shows, very schematically, an example of the productionof a wear-resistant, at least partly uncoated steel part in a schematicillustration. The semifinished part 1, which consists of a steel, forexample a manganese-boron steel, dual-phase steel or TRIP steel, isfirstly fed to surface hardening 2. If a strip-like semifinished part isreeled off a coil 1 a and fed to surface hardening 2, it is, forexample, advantageous to carry out surface hardening, for example in thecase of nitriding, in a continuous hardening furnace at the end ofwhich, for example, the strip-like semifinished part 1, now providedwith a hardened surface, can be wound up on a coil (not shown). Thesurface-hardened strip-like semifinished part is cut to length and fedto hot forming and/or hardening 3, so that process step 3 can produce aformed, at least partly uncoated steel part 4 which is suitable forprocessing, conveying and/or crushing means of agricultural machines,conveying machines, mining machines or building machines. Firstly, thesteel part 4 produced in this way characterizes high strength valuesowing to the hot forming and/or hardening step. Secondly, the surfaceregion of the steel part also has an increased hardening due to thenitriding of the surface which has taken place before hot forming and/orbefore hardening. As indicated above, the process of the inventionenables the decarburization of the surface regions, which takes place toa depth of 100 μm, to be countered by the surface region beingsurface-hardened to a depth of 100 μm or in a region down to a depth of40 μm. The surface hardening is preferably carried out by nitriding.However, carburization of the surface region is also conceivable.

The surface hardening in process step 2 is preferably carried out bymeans of a heat treatment in a heat treatment atmosphere comprising upto 25% by volume of H₂, 0.1-10% by volume of NH₃, H₂O and balance N₂ andalso unavoidable impurities at a holding temperature of from 600° C. to900° C. Reduction of the hydrogen concentration to a maximum of 10% byvolume or limiting of the NH₃ concentration to a maximum of 5% by volumealso leads to a further improvement of the nitriding result.

The depth of the surface hardening can be set via the duration of thesurface hardening, for example at a holding temperature of from 5 s to600 s. The surface is preferably nitrided at a holding temperature offrom 30 s to 120 s, with the temperature being from 680° C. to 840° C.Carrying out the surface hardening before hot forming or hardening hasthe advantage that a heat treatment process can be carried outsignificantly more efficiently using a, for example, strip-likesemifinished part in a continuous hardening furnace or a plate in acontinuous hardening furnace than when using formed steel parts whichhave different shapes and different geometries. The quality of thesurface hardening can likewise be ensured more easily by the use ofstrip-like semifinished parts or semifinished parts configured as ablank.

FIG. 2 then schematically shows a cross section of the semifinished partat three different points in time during the process. At first, thesemifinished part 1 has a more or less homogeneous, for example ferriticmicrostructure 1 a corresponding to the production process, which isdetermined by the combination of production process and steelcomposition. As a result of the surface hardening, the surface region lbis hardened by inward diffusion of nitrogen in the case of nitriding orcarbon in the case of carburization, with the microstructure changingthere. The thickness of the surface region 1 b depends on the durationof the heat treatment. The surface region is usually up to a maximum of100 μm in which the hardness of the semifinished part is altered. Apreferred region, which is a compromise between sufficient surfacehardening and duration of the heat treatment for surface hardening, hasa thickness of from 20 to 40 μm. The duration of surface hardening, forexample in nitriding, is then preferably from 30 s to 120 s. Themicrostructure of the material 1 a remaining underneath the surfaceregion 1 b remains essentially unchanged during the heat treatment.

In the hot forming step, the microstructure of the base material 1 a isthen firstly converted into austenite and, by means of hardening, laterpartially into martensite. In this way, high hardness and goodmechanical strengths are achieved in the base material 1 c. The surfaceregion 1 b remains unchanged except for carburization of these layers.As a result of nitriding, the surface region can continue to remainhardened. In the case of targeted carburization of the surface region 1b instead of nitriding, decarburization can be countered, so that anincrease in the hardness is also achievable here. The formed steel part4 thus has a hardened region 1 b and also a region 1 c which has beenhardened by the hot forming and hardening.

FIGS. 3 and 4 show typical fields of application for the wear-resistant,at least partially uncoated steel part in the form of a conveying screw5 in FIG. 3 and a plowshare 6 for agricultural plows in FIG. 4. Bothcomponents are typical representatives of processing, conveying and/orcrushing means which are used in agricultural machines, conveyingmachines, mining machines or building machines, for example concretemixers, and are exposed to highly abrasive materials. The use of hotformed and/or press hardened steel parts has hitherto not been veryadvantageous because of the increased susceptibility to wear. Due to thesurface hardening of the region which is decarburized during hot formingand/or hardening, the hot forming steels gain an enlarged range of uses.

TABLE 1 Measurement of HV 0.01 Sample A Sample B depth μm (1% NH₃) (4%NH₃) 5 460 546 10 404 490 15 436 447 20 333 415 25 409 394 30 479 453 35453 479 40 436 485 45 492 466

Table 1 shows measurements of the hardness of samples A and B whichconsist of a steel of grade 22MnB5. The samples A and B were subjectedto surface nitriding in a heat treatment atmosphere comprising 1% byvolume of NH₃ or 4% by volume of NH₃ at 760° C. and 90 s in each case.The surface nitriding was carried out at inter-critical temperatures(T>Ac1) since austenite can dissolve more nitrogen than ferrite. Thesamples were subsequently hot formed and hardened. Polished sectionswere made from the hot formed or hardened steel parts and the hardnessHV 0.01 (DIN EN ISO 6507-1) was measured at a distance of 5 μm from thesurface. The microhardness measurement on the samples as a function ofthe content of NH₃ in the heat treatment atmosphere had a greaterhardness at a higher NH₃ content of the heat treatment atmosphere at thesame heat treatment parameters, i.e. hold time and hold temperature.

The hardness of sample A firstly decreases from the value of 460 HVmeasured at the surface to a value of 333 HV at a depth of 20 μm. Thehardness then increases again to a value of about 492 HV, whichindicates that the decarburization of the base material ceases here. Theuppermost region, in particular, from 5 to 15 μm was significantlyhardened by the surface hardening. It can be seen from sample B that thesurface hardening is more pronounced, both in terms of the amplitude andthe depth of hardening, at an increased NH₃ content. This can beattributed to greater diffusion of nitrogen into the surface of thesteel part taking place due to the higher NH₃ concentration in the heattreatment atmosphere. The values for sample B start at 546 at a depth of5 μm and decrease to a value of 394 at a depth of 25 μm. The valuessubsequently increase again to about 466 at a depth of 45 μm. It canclearly be seen that the surface is harder than the base material at adepth of 45 μm.

A similar picture is shown by the measurements on two further examplesshown in FIG. 5 compared to a comparative example. The comparativeexample illustrated by a dotted line displays a reduced hardness below400 HV 1 (DIN EN ISO 6507-1) in the region of 5 to 35 μm. The reductionin the hardness compared to the base material, which is in the rangefrom 450 HV 1 to 500 HV 1, is explained by decarburization during hotforming. The two comparative examples with two different nitridingvariants, once again 1% strength NH₃ heat treatment atmosphere or 4%strength NH₃ heat treatment atmosphere, differ especially in this regionclose to the surface, since hardness of above 500 could be measuredhere. In this way, it is possible, in the case of wear-resistant, atleast partly uncoated steel parts, to provide not only the particularlyhigh tensile strength values of the hot formed and/or hardened steelparts but also a high wear resistance due to greater surface hardness inthe range from, for example, 500 to 700 HV.

1.-12. (canceled)
 13. A method for producing a wear-resistant, uncoatedsteel part for a processing, conveying, and/or crushing mechanism of anagricultural machine, a conveying machine, a mining machine, or abuilding machine from a semifinished part comprising a hardenable steelgrade, the method comprising: heating at least regions of thesemifinished part to a temperature above an Ac1 transformationtemperature; at least partially subjecting the semifinished part tosurface hardening by hardening a surface region to a depth of not morethan 100 microns by heat treating the semifinished part in a heattreatment atmosphere comprising up to 25% by volume of H₂, 0.1-10% byvolume of NH3, H2O and balance N2 and impurities at a hold temperatureof 600-900 degrees Celsius; and at least one of hot forming or hardeningthe semifinished part.
 14. The method of claim 13 wherein hardening thesurface region comprises nitriding or carburizing.
 15. The method ofclaim 13 wherein hardening the surface region comprises maintaining thesemifinished part at the holding temperature for 30 to 120 seconds. 16.The method of claim 13 wherein the surface hardening is performed in acontinuous hardening furnace.
 17. The method of claim 13 wherein thesemifinished part to be surface hardened comprises a manganese-boronsteel or a TRIP steel.
 18. A wear-resistant, uncoated steel part formedof a hardenable steel grade and produced by hot forming and/or hardeninga semifinished part, wherein the wear-resistant, uncoated steel partcomprises a surface region that has been hardened to a depth of not morethan 100 microns by surface hardening by nitriding prior to hot formingand/or hardening the semifinished part, wherein the wear-resistant,uncoated steel part is configured for use as a processing, conveying,and/or crushing mechanism in agricultural machines, conveying machines,mining machines, or building machines, wherein at least regions of thewear-resistant, uncoated steel part that are to be subjected to abrasiveforces have been surface hardened.
 19. The wear-resistant, uncoatedsteel part of claim 18 wherein after hot forming and/or hardening thesteel part has at least the hardness of a base material of the steelpart located under the surface region.
 20. The wear-resistant, uncoatedsteel part of claim 18 wherein the steel part comprises at least one ofa manganese-boron steel, a dual-phase steel, or a TRIP steel.
 21. Thewear-resistant, uncoated steel part of claim 18 wherein the surfaceregion that has been hardened has before hot forming and/or hardening atleast in some regions a hardness of 400-700 HV.